This invention relates to a laminated material in which a segregating membrane is laminated on a supporting material. More specifically, this invention relates to a laminated material of a segregating membrane and a segregating membrane supporting material which is made of non-woven fabric substrate material for supporting and reinforcing a segregating membrane, in the fabrication of segregating membranes such as ultra-filter membranes, precision filter membranes, and reverse osmosis membranes, and to the method of manufacture of that laminated material.
Typically, a segregating membrane, such as a reverse osmosis membrane, is formed by applying a membrane forming liquid, made by dissolving membrane polymer material in a solvent, to a segregating membrane supporting material. Traditionally, thermo-compressed non-woven fabric, made by wet or dry processing methods, is used as segregating membrane supporting material. These segregating membrane supporting materials are made from non-woven fabric; a three-dimensional collection of short fibers joined at fiber crossing points. Naps or fibers extending out from the surface exist for these non-woven fabric supporting materials. Fibers extending out from the supporting material surface are the cause of defects such as pin-holes in the segregating membrane formed on the surface of the supporting material. This is because fibers extending from the supporting material surface pierce through the thin membrane forming liquid when the liquid is applied onto the segregating membrane supporting material.
Fibers extending out from the surface of non-woven fabric segregating membrane supporting material can be suppressed by a processing step to thermo-compress the non-woven fabric surface. By increasing temperature and pressure during the thermo-compression processing step, fibers extending from the surface of the non-woven fabric can be effectively suppressed. This is because the surfaces of thermo-plastic synthetic fibers, which make up the non-woven fabric are compressed. However, as thermo-compression process temperature and pressure are increased, the surface of the segregating membrane supporting material becomes less porous and the density within the non-woven fabric increases. As a result, penetration of the applied membrane forming liquid into the segregating membrane supporting material is obstructed, the anchoring effect responsible for adhesion of the segregating membrane to the supporting material is degraded, and the problem of reduced adhering strength develops.
Penetration of membrane liquid into the non-woven fabric can be improved with thicker non-woven fabric fibers which widen gaps between fibers. However, the total fiber count for non-woven fabric using thicker fibers is lower, and consequently segregating membrane supporting material uniformity decreases and the problem of local vacancies in the membrane liquid results. The supporting material properties of segregating membrane adhesion strength and supporting material uniformity are in mutual opposition, and satisfying requirements for both characteristics is extremely difficult. However, both uniformity and segregating membrane adhesion strength are extremely important supporting material characteristics, and supporting material becomes unusable if either characteristic is bad.
To solve this problem of critical characteristics in mutual opposition, the present applicant has developed a multi-layer segregating membrane supporting material structure, which is a laminate of two non-woven fabric layers as described in previous Japanese Patent Publication No. 4-21526 issued on Apr. 10, 1992. This segregating membrane supporting material is a double-layer structure made up of a coarse exterior surface layer using thick fibers laminated with a fine backside layer using thin fibers.
A laminated segregating membrane supporting material of double-layer construction can achieve a level of segregating membrane adhesion strength within a practical range, and can reduce membrane defects. However, along with recent expansion of segregating membrane applications, membrane adhesion strength capable of withstanding reverse-pressure cleaning is now being required. More robust membrane adhesion strength has come to be demanded, and these demands cannot be satisfied with the physical anchoring effect of prior-art technology.
The present invention was developed to further satisfy requirements for these characteristics. Thus it is a primary object of the present invention to provide a laminated material of a segregating membrane and a segregating membrane supporting material wherein segregating membrane adhesion strength is increased, and in addition a˜uniform segregating membrane can be formed.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.